A composite material is a material of construction comprising two or more integrated materials. The term includes the fiber/polymer composites (FPCs), also referred to as fiber composite plastic or fiber reinforced plastic, which are composed of reinforcing fibers and a polymeric matrix. Examples of reinforcing fibers which can be employed are carbon, glass or aramid fibers. The fiber/polymer composites are referred to, accordingly, as carbon fiber reinforced plastics (CRP), glass fiber reinforced plastics (GRP), and aramid fiber reinforced plastics (ARP).
In the aerospace sector, alloy metal such as aluminum is being replaced more and more by such fiber/polymer composites. The advantage of these materials lies in their weight, which is lower in relation to alloy metal and steel, and also in their high specific strength and stiffness.
The requirements imposed on aircraft coatings are particularly exacting. Thus the requirement of safety is much more stringent than it is, for example, in the automobile segment. Moreover, the coatings are to protect the substrate for a number of decades, are to overcome ambient conditions entailing frequent fluctuations, such as extreme pressure and temperature fluctuations, for example, and also elevated UV exposures, and to satisfy esthetic requirements, such as high gloss and good leveling. The outer skin comes into contact with a variety of service fluids and auxiliaries, such as kerosene or oils, and also with de-icing products, and must be protected accordingly by the paint system. Furthermore, the coatings ought to adhere well to the substrate. Principally, in the case of fiber/polymer composites (FPCs) which have a high electrical resistance, moreover, additional requirements are imposed on the surface, in order to achieve improved dissipation of excessive electrical charges, in order, for example, to distribute the energy after a lightening strike. Therefore, an aircraft coating applied to FPC ought to have at least one coating which allows the resistance of the surface to be reduced and electrostatic dissipation to be achieved. This function may be obtained, for example, by virtue of an antistatic coating.
In aircraft engineering there are national and international standards setting threshold values for the paint systems. The surface resistance, for example, ought typically to have a value of around 106 ohm.
The surface resistance (also called leak resistance) provides information on the insulation state prevailing on the surface of a coating, or the propensity of the nonconductor to form a conducting surface layer. The surface resistance may be altered by external influences such as moisture, acid, etc. It can be determined, for example, with the aid of a reed comb electrode (Römpp Lexikon Lacke and Druckfarben, Georg Thieme Verlag Stuttgart 1998, ISBN 3-13-776001-1, entry heading “Oberflächenwiderstand”).
In aircraft engineering, a metal substrate is typically pretreated in order to remove impurities completely. FPCs, in contrast, are usually not pretreated. In particular there is no chemical pretreatment with, for example, organic solvents, so as to prevent swelling of the material. The only suitable methods would be physical methods such as corona discharge, for example. Instead, FPCs are coated with an in-mold primer, before then being provided with a primer-surfacer as a priming coat. Thereafter a topcoat finish is applied. This topcoat finish comprises basecoat and clearcoat. Depending on requirement and on customer stipulation, the in-mold primer and parts of the topcoat system may be omitted. In OEM aircraft engineering, the individual parts are in many cases already individually coated. Following final assembly, there are, first of all, test flights, before the final paint finish, usually a topcoat, is applied. Because of the characteristics of the substrate, the paint system—both in OEM aircraft engineering and in maintenance—cannot be subjected to a thermal cure. The melting temperatures or glass transition temperatures may be situated in ranges as low as about 70° C. Also, in view of the dimensions of a fully assembled aircraft or its component parts, no thermal cure is possible.
U.S. Pat. No. 4,155,896 discloses an antistatic, nonaqueous coating composition for aircraft. The composition there is suitable for coating aluminum substrates.
WO2008/085550 describes electrically dissipating coating compositions for aircraft, which are applied to commercial primer-surfacers, which prevent layers of ice forming.
Patent application DE19948821 describes electrically conductive hydroprimers for plastics, including fiber reinforced plastics. These plastics, however, are not applied in aircraft engineering or in space travel.
Now, though, the aim more and more is to reduce the fraction of solvents and to provide water-based coating systems.
It was an object of the present invention to meet the requirements described above that are imposed on a primer-surfacer for the aerospace sector. The intention was to provide a waterborne composition which as a primer-surfacer is able to meet the exacting requirements imposed on an aircraft paint system and at the same time can be employed as an antistatic coating. The composition ought to cure at temperatures of less than 40° C., and ought to exhibit effective adhesion both to fiber/polymer composites directly and to fiber/polymer composites coated with in-mold primers. Furthermore, the primer-surfacer ought to exhibit good leveling and ought to be able to be applied at a high film thickness.